The present invention relates to a method for polishing a slight region of the surface of a workpiece with an extremely high precision, such as a lens or an optical element, and to a tool for carrying out the method.
The surfaces of nonspherical lenses and X-ray optical elements incorporated in electronic instruments or optical instruments are polished in manufacturing them. There is a demand for a method for precisely processing areas of 0.01 .mu.m and smaller. In order to comply with such a demand, it is necessary to develop a method for precisely polishing a slight region of the entire surface of a workpiece.
Conventionally, a polishing process or a lapping process has been adopted to abrade or polish the workpiece with high precision. But it is impossible for the conventional methods to precisely polish areas of 0.01 .mu.m and smaller. In recent years, a magnetic polishing method which uses magnetic polishing fluid as a polishing material is noteworthy in that the method is capable of polishing with high precision. The magnetic polishing fluid refers to magnetic fluid alone or a mixture comprising magnetic fluid and a polishing material of fine grain which is dispersed and suspended in the magnetic fluid.
According to the magnetic polishing method, the magnetic polishing fluid is supplied between the polishing portion disposed at the leading end of a polishing tool and a workpiece and a magnetic field is applied therebetween. As a result, the magnetic polishing fluid is magnetically held between the polishing portion and the surface of the workpiece pressurized by the magnetic polishing fluid. A high speed rotation of the polishing tool causes the magnetic polishing fluid to rotate at a high speed. Thus, the magnetic polishing fluid polishes the surface of the workpiece. In order to improve the polishing performance of this method, the direction and magnitude of the magnetic field are varied to change the pressure applied by the magnetic polishing fluid to the surface of the workpiece and control the motion of the magnetic polishing fluid. The magnetic polishing method is disclosed in Japanese Laid-Open Patent Publication No. 60-118466 and No. 61-244457, and Japanese Patent Publication No. 1-16623.
According to the magnetic polishing method, a magnetic coercive force allows the polishing material to intensively act on a slight region of the surface of the workpiece. Thus, the surface of the workpiece may be polished with a higher precision than with the conventional polishing method previously described. However, it is difficult for the magnetic polishing method to precisely polish the workpiece in an area of 0.01 .mu.m or smaller.
That is, according to the magnetic polishing method, the magnetic polishing fluid is rotated at a high speed by the high speed rotation of the polishing tool. Therefore, the polishing performance is determined by the rotational state of the polishing tool. That is, the number of rotations of the polishing tool is liable to vary or the shaft thereof is liable to dislocate from its axis. Thus, the amount of abrasion is different from piece to piece; the polishing operations are non-uniform; and the surface polishing may be dislocated from the surface which is desired to be polished. There are spacial allowances in the operation mechanism, e.g. a rotary mechanism, so that each member is capable of carrying out a smooth motion. Consequently, the polishing portion disposed at the leading end of the polishing tool unavoidably undergoes a small amount of movement. Therefore, it is inevitable that the surface of the workpiece is polished uniformly throughout the surface. That is, so long as the polishing portion is rotated at a high speed, it is impossible to prevent the occurrence of the above-described problems.
According to the above-described conventional magnetic polishing method carried out by rotating the polishing tool at a high speed, the force for pressing the magnetic polishing fluid against the surface of the workpiece is generated not by the rotation of the polishing tool but by the application of a magnetic field. Therefore, unless a strong magnetic field is applied, a sufficient polishing force is not generated and the amount of abrasion varies depending on the magnitude of the magnetism. Consequently, according to this method, it is necessary to provide the polishing apparatus with a large-scale magnetism generating means such as an electro-magnet and to strictly control the magnetic force. In addition, it is necessary for the workpiece to constitute a part of a magnetic circuit for the generating magnetism. That is, the workpiece needs to be composed of a magnetic substance except in the case of a thin workpiece. When the surface of thin workpiece is polished, the force to be applied by the magnetic polishing fluid to the surface of the workpiece is changed due to a slight fluctuation in the thickness of the workpiece or due to a change in magnetic properties. Thus, it is difficult to control the amount of abrasion or the polishing precision. This method may not be applied to a lens or an optical element because the lens and the optical element are made of nonmagnetic material and are considerably thick.
In order to overcome the problems of the above-described conventional magnetic polishing method, the present inventors developed a method for polishing a slight region of a workpiece and a polishing tool for carrying out the method (see U.S. Ser. No. 07/708,867, filed May 31, 1991, by Shinichi Mizuguchi et al.). According to the method, the workpiece may be polished without being influenced by the magnetic properties thereof and may be preferably applied to a workpiece made of a nonmagnetic material. More specifically, an actuator (or actuators) comprising a piezoelectric element moves the polishing portion slightly in an XY-direction parallel with the surface of the workpiece and/or a Z-direction perpendicular to the surface of the workpiece. The slight movement of the polishing portion is transmitted to the magnetic polishing fluid to polish the surface of the workpiece, with pressure mechanically applied to the space between the polishing portion and the workpiece.
According to this novel polishing method and the polishing tool therefor, it is unnecessary to rotate the polishing portion at a high speed. In addition, it is unnecessary for the workpiece to constitute a component of a magnetic circuit in order to hold the magnetic polishing fluid between the workpiece and the polishing portion and apply pressure to the magnetic polishing fluid. Thus, this method is capable of solving the problems of the conventional polishing method carried out by rotating the polishing tool at a high speed.
According to this polishing method of the present inventors, the Z-direction actuator comprising the piezoelectric element presses the magnetic polishing fluid against the workpiece through the polishing portion and moves the polishing portion slightly along the surface of the workpiece. Thus, the surface of the workpiece is polished. That is, the XY-direction actuator or the Z-direction actuator moves the magnetic polishing fluid slightly in a horizontal and/or vertical direction with respect to the surface of the workpiece, and the Z-direction actuator cause the magnetic polishing fluid to be mechanically pressed against the surface of the workpiece.
According to this method, it is unnecessary to rotate the polishing portion. Therefore, the number of rotations of the polishing tool does not vary or the shaft thereof does not dislocate from the axis thereof. As a result, the surface of the workpiece may be abraded to approximately the same depth and the surface roughness is the same throughout the surface. The actuator comprising the piezoelectric element has neither mechanical sliding portions nor mechanical operating mechanisms and as such accurately operates according to an applied voltage. Therefore, the motion of the magnetic polishing fluid is stable. In this respect, the surface of the workpiece may be abraded to approximately the same depth and degree throughout the surface thereof. The area of movement of the magnetic polishing fluid made by the XY-direction actuator is much smaller than that of the rotary motion adopted by the conventional method described previously. Thus, the surface of the workpiece may be finely polished to a mirror-like surface finish.
According to this polishing method, pressure for mechanically pressing the magnetic polishing fluid against the workpiece may be applied by the Z-direction actuator comprising the piezoelectric element. Therefore, it is unnecessary to provide the polishing apparatus with a magnetic circuit connecting the polishing portion to the workpiece. Accordingly, a workpiece made of a nonmagnetic substance may be abraded and the pressure to be applied to the workpiece may be controlled by a voltage to be applied thereto irrespective of the difference in the magnetic properties of the workpiece caused by the material quality of the workpiece and the thickness thereof. Thus, the material quality of the workpiece and the configuration thereof do not affect the abrasive efficiency or the abrasive precision of this method. In addition to a workpiece made of a magnetic substance such as steel which may be polished by the above-described conventional polishing method, this method is capable of polishing a workpiece made of a nonmagnetic substance such as glass or ceramic which may not be polished by the above-described conventional polishing method. Further, this method polishes the workpiece irrespective of its thickness and to a plane, spherical or free-curved configuration.
According to this method, the polishing portion moves slightly along the surface of the workpiece by applying a voltage to the piezoelectric element. Thus, the magnetic polishing fluid is capable of polishing the workpiece by polishing a slight region with the polishing portion at high precision. As a result, the entire surface of the workpiece may be polished to approximately the same depth and quality by performing such a polishing operation over the whole surface of the workpiece. This method is capable of polishing the workpiece much more uniformly, to a mirror-like surface finish, and in an area of 0.1 .mu.m, compared with the conventional polishing method described previously, with ease and reliability.
However, later research carried out by the present inventors revealed that the method and the tool therefor have the following disadvantages.
That is, it is difficult for the method to accurately control the amount of abrasion and then make use computer controlled polishing. In addition, the surface roughness is different from piece to piece.